Mitochondrial outer membrane protein which regulates mitochondrial fission/fusion dynamics (PubMed:21701560, PubMed:23921378, PubMed:33632269). Promotes the recruitment and association of the fission mediator dynamin-related protein 1 (DNM1L) to the mitochondrial surface independently of the mitochondrial fission FIS1 and MFF proteins. Regulates DNM1L GTPase activity and DNM1L oligomerization. Binds ADP and can also bind GDP, although with lower affinity. Does not bind CDP, UDP, ATP, AMP or GTP. Inhibits DNM1L GTPase activity in the absence of bound ADP. Requires ADP to stimulate DNM1L GTPase activity and the assembly of DNM1L into long, oligomeric tubules with a spiral pattern, as opposed to the ring-like DNM1L oligomers observed in the absence of bound ADP. Does not require ADP for its function in recruiting DNM1L. {ECO:0000269|PubMed:21508961, ECO:0000269|PubMed:21701560, ECO:0000269|PubMed:23283981, ECO:0000269|PubMed:23530241, ECO:0000269|PubMed:23921378, ECO:0000269|PubMed:24515348, ECO:0000269|PubMed:29083303, ECO:0000269|PubMed:33632269}. Assembly factor involved in the biogenesis of the mitochondrial-specific ribosomes (mitoribosomes) (PubMed:28892042, PubMed:30215512, PubMed:31666358). Specifically associates with intermediates of the mitochondrial ribosome large subunit (mt-LSU) and is required for proper ribosome assembly, possibly preventing premature association of the large and small ribosomal subunits (PubMed:28892042, PubMed:30215512, PubMed:31666358). Thereby, indirectly regulates mitochondrial translation (PubMed:28892042, PubMed:30215512, PubMed:31666358). It is also required for complete assembly of the mitochondrial respiratory chain complex I (PubMed:31666358). May also function in DNM1L-mediated mitochondrial fission (PubMed:29083303). {ECO:0000269|PubMed:28892042, ECO:0000269|PubMed:29083303, ECO:0000269|PubMed:30215512, ECO:0000269|PubMed:31666358}. This is the function of MIEF1 (mitochondrial elongation factor 1, Ensembl gene identifier ENSG00000100335).